`match_results` has different parameter type compared to `std::match_results`

[zufuliu]

this cause warnings when porting std::regex code to boost::regex: warning C4267: 'argument': conversion from 'size_t' to 'int', possible loss of data.

difference_type length(int sub = 0) const
https://github.com/boostorg/regex/blob/develop/include/boost/regex/v5/match_results.hpp#L100
https://en.cppreference.com/w/cpp/regex/match_results/length

string_type str(int sub = 0) const
https://github.com/boostorg/regex/blob/develop/include/boost/regex/v5/match_results.hpp#L164
https://en.cppreference.com/w/cpp/regex/match_results/str

const_reference operator[](int sub) const
https://github.com/boostorg/regex/blob/develop/include/boost/regex/v5/match_results.hpp#L199
https://en.cppreference.com/w/cpp/regex/match_results/operator_at

I think it's better to use size_type sub as parameter as difference_type position(size_type sub = 0) const:
https://github.com/boostorg/regex/blob/develop/include/boost/regex/v5/match_results.hpp#L131
https://en.cppreference.com/w/cpp/regex/match_results/position

and C-style cast inside these int sub functions can be avoided:

- if(sub < (int)m_subs.size() && (sub > 0))
+ if(sub < m_subs.size())

[zufuliu]

This seem can't be fixed as MSVC can't disambiguates operator [](const wchar_t *) const and operator [](unsigned __int64) const
for search_base = position = m_result[0].second; inside perl_matcher_common.hpp.